Questions for the experts...

I have several questions that I can't seem to be able to resolveâ
Could you please help me out?

First, there is the issue of frequency. How often should one exercise
each body-part? Most agree that the absolute minimum should be 3 times
a week. But is that a minimum for exercising in general or for each
individual muscle group?

Assuming that one could theoretically exercise each and every day,
what would be the recommended frequency? 

There are theories that the maximum should be 4 times a week, and that
it should be broken up to 2 couples of consecutive days with a day in
the middle when one doesn't even go near a gym. But even then, there
are systems where one exercises all muscle groups every day, creates
two sets of muscle groups and exercises one set per day alternating
between the two (getting a complete workout in two days) and where
only one muscle group is exercised each day. 

How does one figure out what is the best system for them and what is
the criterion? Is muscle soreness any indication? Or do different
systems do different things for each person? If that is the case, how
long should someone follow a system before deciding what that system
does for them?

And then there is the issue of intervals between sets. It is evident
that different intervals have different results. It seems for example,
that those striving for definition should exercise with smaller
intervals and that those striving for muscle mass should exercise with
longer ones. But even then the durations given for the intervals vary
considerably. 

How does one figure out what interval works for them? Is the interval
depended on the size of the muscle exercised, with bigger muscles
requiring longer intervals? 

Is heart rate any indication? Assuming one has a heart rate monitor,
could they calculate their intervals according to their heart rate
(meaning, drop your heart rate to this number before you go again if
you want definition and to that number if you want muscle mass)? Or is
heart rate not important and intervals intend to give the time to the
muscle to do other things that are not measurable that way?

Those are my questions for the time being.

Thank you!